Unit fuel injectors operated by cams, have long been used in compression ignition internal combustion engines for their accuracy and reliability. A unit injector typically includes an injector body having a nozzle at one end and a cam driven injector plunger mounted for reciprocal movement within the injector body. When the cam causes the plunger to reach its retracted position, a controlled amount of fuel is metered into the injector such that upon plunger advancement the metered quantity of fuel is forced through orifices in the injector nozzle into a combustion chamber of the engine. To achieve optimal engine operation, the fuel must be injected at very high pressure to achieve the maximum possible atomization of the injected fuel. In addition, the interval of injection needs to be carefully timed during each cycle of injector operation in dependance upon the movement of the corresponding engine piston. Fuel system designers have found that both high pressure and carefully timed operation can normally be achieved most easily by mounting the injector cam on the engine camshaft which rotates in a fixed relationship with the engine crankshaft. An injector drive train extending between the injector cam and injector plunger is adapted to convert cam rotation into reciprocating movement of the injector plunger. A typical type of cam operated unit injector is disclosed in U.S. Pat. No. 3,544,008 assigned to the same assignee as the subject invention.
Many unit injector systems modify the fuel injector or injector drive train to vary the rate of fuel injection during each injection interval in order to achieve still further improvement in overall engine operation. One such system is disclosed in U.S. Pat. No. 3,965,875 to Perr, one of the co-inventors of this invention. The '875 patent discloses a fuel injection system providing a slower rate of fuel injection during the initial phase of fuel injection. In the '875 system, an injection rate control device is connected in line with a conventional injector train and includes an auxiliary spring having a lower spring rate than that of the remaining portion of the injection train to slow the plunger advance and the resulting initial rate of fuel injection during the initial portion of each injection interval. However, the '875 patent does not specifically discuss the concept of a cam profile that creates carefully modulated injector plunger velocities.
U.K. Patent No. 318,889 discloses the concept of an injector cam profile having successive sections of differing shape to control the rate of fuel delivery at slow engine speeds. As engine speed decreases, the injection arc on the cam actuating the injector is adjusted to a steeper section of the fuel cam, thereby delaying the commencement of injection. This approach maintains sufficient injection velocity and acceptable atomization at lower engine speeds, but does not address the need for an idealized schedule of fuel flow rates throughout each injection interval at both high and low engine speeds.
Although the systems disclosed in the above references create different stages of injection, improvement and further refinement of the staged injection cycle is still desirable. For example, neither of the cams disclosed in the above references are necessarily able to adequately and simultaneously control engine noise, nitric oxide emissions, and unburned hydrocarbons.
Herdin et al. U.S. Pat. No. 4,355,686 and U.K. Patent No. 277,678 disclose other types of known injector cams having shapes designed to achieve specific functional results. These patents fail to disclose an injector cam profile which improves engine operation throughout the entire operating range of the engine. There is a need, unfulfilled in the prior art, for an injector cam having a profile that improves engine operation and the injection characteristics throughout the entire operating range of the engine, at both high and low operating speeds.